Time integrating pixel sensor

ABSTRACT

A time-integrating pixel sensor having a photo-detector, a capacitor, a comparator and a pixel data buffer. In operation, the photo-current from the photo-detector charges the capacitor and produces a photo-voltage. The photo-voltage sensed by the capacitor and a reference voltage is compared with the comparator. If the photo-voltage exceeds the reference voltage, a global code value is latched into the pixel data buffer. The optical power falling on the photo-detector is determined from the latched code value. An array of sensors is incorporated into a semiconductor device together with circuitry to read and decode the pixel data buffers. The reference voltage may be varied in time to increase the dynamic range of the sensor.

TECHNICAL FIELD

[0001] This invention relates generally to techniques and apparatus forimage sensing, and more particularly to a method and apparatus forobtaining a digital measurement of a photo-current.

BACKGROUND OF THE INVENTION

[0002] A major problem in imaging is the limited dynamic range of imagesensor arrays. As a result, some portions of the image may be saturatedwhile other portions are under exposed. A typical photodiode opticaldetector, such a charge-coupled device (CCD), consists of an array of pnjunction photodiodes. Each photodiode has a capacitance associated withit, and when light falls on the detector the resulting photo-currentcharges the capacitance. The charge is thus the time integral of thelight intensity falling on the detector. The CCD periodically andsequentially switches the charge to a video line, resulting in a seriesof pulses, which can be converted into a voltage signal representing thelight pattern incident on the array. If the integration time is toolong, the device will saturate. If the integration time is too short,the voltage signal will be lost in the noise of the device.

[0003] One approach to adjusting sensitivity is therefore to adjust theintegration time according to the intensity of light falling on thearray. This can be done globally for the whole array by adjusting theswitching rate, for example. This allows the array to adjust for variouslight levels (as in adjusting the exposure time of an analog camera),but does not increase the dynamic range within a single image.

[0004] Another approach is to measure the time taken for the capacitorvoltage to reach a reference level. An example of such a system is givenin “Intensity Mappings Within The Context Of Near-Sensor ImageProcessing”, by Anders Åström, Robert Forchheimer and Per-ErikDanielsson, IEEE Transactions on Image Processing, Vol. 1, No. 12,December 1998. A large time indicates a low intensity of light fallingon the array, while a short time indicates a high intensity. The voltagereference level is common to the whole array, but may be adjustedaccording to the overall intensity falling on the array. This approachis computationally intensive, since the integration time is measured byinterrogating the output from a comparator at predetermined intervals.These intervals need to be very close together for accurate measurement.This approach also requires each pixel to have a readout circuit. Inaddition, each sensor element requires a precharge transistor.

[0005] A further device called a “Locally Autoadaptive TFA Sensor” isdescribed in “High Dynamic Range Image Sensors In Thin Film On AsicTechnology For Automotive Applications”, by M. Böhm et al., in AdvancedMicrosystems for Automotive Applications, D. E. Ricken, W. Gessner(Eds), Springer Verlag, Berlin, pp 157-172, 1998. In this device, theintegration time is adapted for each individual pixel according to thelocal illumination intensity. The capacitor voltage is compared after aninitial integration time to a reference voltage that is slightly lessthan half of the saturation voltage. If the voltage is less than thereference voltage, the integration time is doubled. This process isrepeated until the reference voltage is exceeded. Thus, saturation ofthe capacitor is avoided and the integration time is extended to avoidunder exposure. The final voltage and the integration time are togetherused to determine the illumination intensity. This approach requirescircuitry to compare and adjust the integration time and circuitry tomeasure the voltage at the end of the integration time.

[0006] In light of the foregoing discussion, it can be seen that thereis still a need in the art to have image sensor arrays resulting in animage representative of the intensity distribution of an object suchthat portions of the image are not saturated or underexposed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The features of the invention believed to be novel are set forthwith particularity in the appended claims. The invention itself however,both as to organization and method of operation, together with objectsand advantages thereof, may be best understood by reference to thefollowing detailed description of the invention, which describes certainexemplary embodiments of the invention, taken in conjunction with theaccompanying drawings in which:

[0008]FIG. 1 is a block diagram of a photodiode sensor, in accordancewith an aspect of the present invention.

[0009]FIG. 2 is a graph of showing photo-voltage as function ofintegration time for a photodiode sensor, in accordance with an aspectof the present invention.

[0010]FIG. 3 is a block diagram of a system utilizing a photodiodesensor array, in accordance with an aspect of the present invention.

[0011]FIG. 4 is a flow chart showing a method of sensing optical powerin accordance with the present invention.

[0012]FIG. 5 is a flow chart showing a further method of sensing opticalpower in accordance with the present invention.

[0013]FIG. 6 is a graph showing photo-voltage as function of integrationtime for a photodiode sensor, in accordance with an aspect of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0014] While this invention is susceptible of embodiment in manydifferent forms, there is shown in the drawings and will herein bedescribed in detail specific embodiments, with the understanding thatthe present disclosure is to be considered as an example of theprinciples of the invention and not intended to limit the invention tothe specific embodiments shown and described. In the description below,like reference numerals are used to describe the same, similar orcorresponding parts in the several views of the drawings.

[0015] Referring now to FIG. 1, a time-integrating pixel architecture100 of a preferred embodiment of the invention is shown. The pixelarchitecture 100 comprises a photo-detector 102 and a floating diffusioncapacitor 104 with capacitance C_(fd). In operation, the photo-currentcharges the floating diffusion capacitor and produces a photo-voltage106. The charge is allowed to move directly from the photo-detector tothe diffusion capacitor without the use of a transfer gate. Thephoto-voltage 106, sensed by the floating diffusion capacitor, and areference voltage 108 are supplied as inputs to comparator 110. If thesystem is reset at time t=0, the voltage V across the capacitor at timet is the time integral of the photo-current i, which is given by${V = {{\frac{1}{C_{fd}}{\int_{0}^{i}{{i(\tau)} \cdot \quad {\tau}}}} = \frac{it}{C_{fd}}}},$

[0016] where C_(fd) is the capacitance of the floating diffusioncapacitor and t is the integration time of the capacitor. Thephoto-current is given by ${i = \frac{{PeQE}\quad \lambda}{hc}},$

[0017] where P is the illumination intensity, q is the charge on anelectron, λ is the wavelength of the light, c is the speed of light, his Planck's constant and QE is the quantum efficiency of thephoto-detector. Hence the voltage across the capacitor is related to theintegration time${V = {\frac{{PqQE}\quad {\lambda t}}{C_{fd}{hc}} = {\frac{P}{K}t}}},$

[0018] where $K = \frac{C_{fd}{hc}}{{qQE}\quad \lambda}$

[0019] is a constant. For an illumination intensity P_(i), the time toreach the reference voltage V_(ref) is$t_{i} = {K{\frac{V_{ref}}{P_{i}}.}}$

[0020] That is, after an integration time t_(i) the photo-voltage 106sensed by the floating diffusion capacitor and the reference voltage 108are equal. In the pixel architecture shown in FIG. 1, a global counteror code generator 118 is reset at time t=0. When the photo-voltage 106exceeds the reference voltage 108 for a particular pixel, the output 112from the comparator 110 changes value. At that time the global countervalue or code value is latched and stored in the associated pixel databuffer 114. Latching the counter or code values avoids the need tointerrogate the comparator output. In the preferred embodiment, thecounter or code value is stored as a 10-bit value in the pixel databuffer 114. However, different size buffers may be used. Generally,larger buffers are more expensive but provide greater accuracy. Onelatch is used for each bit in the buffer and is assumed in the figure tobe an integral part of the buffer.

[0021] The apparatus shown in FIG. 1 provides a direct digital value(the counter or code value) related to the illumination intensity. Thisis in contrast to prior systems in which an analog voltage is sensed andthen converted to digital value using an analog-to-digital converter.The simplicity of the system of the present invention compared to priorsystems allows for sensors that are both smaller and cheaper.

[0022] In FIG. 1, only a single pixel sensor is shown. However, it isenvisioned that an array of pixel sensors will be integrated in a singledevice, such as a CMOS device, may be utilized in conjunction with thepresent invention. Pixel data memory decoder 116 is used to decode thedata from individual pixels sensors and send it to a processor (notshown).

[0023]FIG. 2 is a graph showing photo-voltage as a function ofintegration time for a photodiode sensor of the present invention. Inthis embodiment the reference voltage is varied as a function of time.Two examples of the time-variation of the reference voltage are shown inFIG. 2. The first is denoted by the dashed line labeled “LinearMapping;” the second is denoted by the line labeled “Nonlinear Mapping”.“Linear Mapping” may be chosen to provide a linear relationship betweenthe stored digital codes and the power. When Nonlinear Mapping is usedthe dynamic range of the sensor is maximized. Nonlinear Mapping is usedin the examples described below. The line labeled P₁ shows therelationship between integration time and photo-voltage for light withoptical power P₁. In the preferred embodiment, the global counter orcode generator (118 in FIG. 1) is set to zero at time t=0. At time t₁,the photo-voltage reaches the reference voltage V_(ref), and the outputfrom the comparator (110 in FIG. 1) switches value. At this time, theglobal counter is latched into the pixel data buffer (114 in FIG. 1). Ata later time, the photo-voltage will reach the saturation level V_(sat).The line labeled P₂ shows the relationship between integration time andphoto-voltage for light with optical power P₂, which is less than P₁. Attime t₂, the photo-voltage reaches the reference voltage V_(ref), andthe output from the comparator (110 in FIG. 1) switches value. At thistime, the global counter is latched into the pixel data buffer (114 inFIG. 1). Thus, when light with a lower optical power falls on thephoto-detector, the integration time taken to reach the referencevoltage is increased. The integration time taken to reach the referencevoltage is therefore inversely proportional to the intensity or theoptical power. Similarly, the line labeled P₃ shows the relationshipbetween integration time and photo-voltage for light with optical powerP₃, which is less than P₁ or P₂. At time t₃, the photo-voltage reachesthe reference voltage Vref, and the global counter value is latched intothe pixel data buffer. If the voltage reference is increased, a longerintegration time will be required. If the voltage reference isdecreased, a shorter integration time will be required. Even when nolight is incident upon the photo-detector, small currents and voltageswill be present in the sensor because of electrical noise. Voltagelevels below the voltage noise floor will be undetectable, so theminimum voltage reference level is determined by the voltage noisefloor. There is an upper limit to the amount of charge that can bestored by the capacitor. The voltage corresponding to this maximumcharge is called the saturation voltage. The maximum voltage referencelevel V_(max) is determined by the saturation voltage.

[0024] The integration time is inversely proportional to the intensity,and the optical power or intensity is given by$P_{i} = {K{\frac{V_{ref}}{t_{i}}.}}$

[0025] The integration time t_(i) may be determined directly from theglobal counter value if the global counter value is set to zero at thestart of the integration time. Otherwise, the integration time may bedetermined from the difference between the counter value at the end ofthe integration N_(i) and the counter value N₀ at the start of theintegration. The optical power is given by$P_{i} = {C\frac{V_{ref}}{N_{i} - N_{0}}}$

[0026] where C=K'f_(counter) is a constant and f_(counter) is thecounter frequency. This computation is performed by the processor. Pixeldata memory decoder 116 is also used to couple the global counter orcode generator 118 to the individual pixel sensors.

[0027] The minimum intensity that can be measured is${P_{\min} = {K\frac{V_{ref}}{T_{frame}}}},$

[0028] where T_(frame) is the time between consecutive video frames, orthe maximum allowable integration time. Below this intensity, thecounter will not be latched within the frame period.

[0029] The saturation intensity is${P_{\max} = {K\frac{V_{ref}}{T_{clock}}}},$

[0030] where T_(clock) is the counter period or the minimum allowableintegration time. Above this intensity, the counter will be latched in asingle counter cycle. Hence, if the voltage is constant, the dynamicrange within a single image is${DR} = {{20 \cdot {\log_{10}\left( \frac{T_{frame}}{T_{clock}} \right)}}{{dB}.}}$

[0031] The intraframe dynamic range of an image may be adjusted byvarying the reference voltage V_(ref) during a single frame. As shown inFIG. 2, to obtain the maximum possible dynamic range V_(ref) may varysuch that it equals V_(max) at time equaling T_(clock) and equalsV_(min) at time equaling T_(frame). The voltage V_(ref) can be variedusing a digital-to-analog converter (DAC) to supply the referencevoltage 108. If the maximum reference voltage is V_(max) and the minimumis V_(min), the overall dynamic range is${DR} = {{20 \cdot {\log_{10}\left( \frac{V_{\max}T_{frame}}{V_{\min}T_{clock}} \right)}}{{dB}.}}$

[0032] For example, for a 50 Hz frame rate and a 10 MHz clock, themaximum dynamic range is 106 dB if V_(ref) is constant. If, in addition,the reference voltage is varied in the range 300 μV-1.5V, and anadditional 74 dB of dynamic range is added, this gives a total range of180 dB.

[0033] The accuracy of the sensor is determined by how much thephoto-voltage can change within one counter cycle. If the voltagereference is exceeded before the n^(th) counter cycle at timet=(n−γ)T_(clock), where γ<1, the counter value is latched at the valuen, and the error in the intensity is given by${error} = {{{K\frac{V_{ref}}{\left( {N - \gamma} \right)T_{clock}}} - {K\frac{V_{ref}}{{nT}_{clock}}}} = {{K\gamma \quad \frac{V_{ref}}{{n\left( {n - \gamma} \right)}T_{clock}}} = {\frac{\gamma \quad P_{i}}{n}.}}}$

[0034] The relative error is therefore inversely proportional to n.Large intensities will have the largest relative error. The error can bereduced by making n larger. This, in turn, can be achieved by increasingthe clock (counter) speed or by raising the reference voltage.

[0035] Referring to FIG. 3, the individual pixel sensors are preferablyarranged in an array 302. Each element of the array comprises aphoto-detector, a floating diffusion capacitor electrically coupled tothe photo-detector and a comparator. The comparator has a first inputfor receiving a reference voltage, a second input electrically coupledto the floating diffusion capacitor and an output. Each element alsoincludes a pixel data buffer, which may be a random access memory (RAM).The pixel array is addressed in a manner similar to that used to addressstandard memory (DRAM, SRAM, etc). Each column of pixels is preferablyconnected to its own data bus. A row address is sent to a pixel rowaddress decoder 304 that selects a particular row of pixels using wordlines 314. Pixel column address decoder 306 also includes a data decoderthat may be used to receive data from or transmit data to selectedcolumns of pixels, according to the selected row. This allows individualpixels to be addressed, so that data can be “written into” or “read outof” the pixels via pixel bit lines 316. A global sequencer 312 iscoupled to the system controller 310. The global sequencer generateslookup table addresses 324 and 334 that are passed to lookup tables 326and 334. Lookup table 1, 326, is used to convert the lookup tableaddresses 324 into voltage reference values that are passed to a digitalto analog converter (DAC) 328. The DAC 328 in turn generates thereference voltage 108 that is passed to the pixel array 302. Lookuptable 2, 334, is used to convert actual counter values into systemappropriate digital code values that are passed to the pixels via thepixel column decoder 306. The digital code values need not be linearlyrelated to the actual counter values. The digital code values maycorrespond to the optical power level associated with each counter valuefor a given voltage reference. This avoids the need to compute theoptical power levels later. In addition, a pixel timing controller 308is coupled to the row and column decoders to control the timing ofaccess to the individual elements of the array. The pixel bit data issensed by the column decoder 306 via bit-lines 316 and the data is sentas a multiplexed data stream 322 to the image processor and systemcontroller 310. The image processor performs the conversion fromintegration time to optical power for each of the pixel sensors and mayperform other image processing functions, such as color processing orcompression. Data representing the processed image is output from theimage processor at 318. The conversion may be performed via a lookuptable or via a calculation or by a combination thereof. The systemcontroller, included in 310, provides pixel array timing and controlsignals 320 that are passed to a pixel timing and control unit 308. Thepixel timing and control unit 308 controls the pixel row decoder 304 andthe pixel column decoder 306. The system controller, included in 310,also provides control of the global counter or code generator 312.

[0036] A flow chart depicting one embodiment of the method of thecurrent invention is shown in FIG. 4. Operation begins at start block402. The pixel sensor is reset at block 404, by discharging the floatingdiffusion capacitor for example. The output from the photo-detector isintegrated at block 406. If a floating diffusion capacitor is used, thephoto-detector output current is integrated by the capacitor to producea photo-voltage. At decision block 408 the photo-voltage is comparedwith a reference voltage. If the photo-voltage is less than thereference voltage as depicted by the positive branch from decision block408, the current value of a global counter or code generator (which issupplied to the pixel sensor) is stored into a pixel data buffer atblock 410. Flow then continues to decision block 412. If thephoto-voltage is greater than the reference voltage, as depicted by thenegative branch from decision block 408, the flow continues to block412. Decision block 408 may be implemented as a comparator. Thus thelast value stored in the pixel data buffer will be the counter value atwhich the photo-voltage reaches the reference voltage. The value is thuslatched into the pixel data buffer. At decision block 412, a check ismade to determine if the maximum time for integration has been exceeded.If the maximum time has been exceeded, as depicted by the positivebranch from decision block 412, the pixel data buffer is read. If themaximum time has not been exceeded, as depicted by the negative branchfrom decision block 412, the system waits until the time has beenexceeded. At block 414, the pixel data buffer is read, whether or notthe reference voltage has been reached. At block 416, the optical poweris determined from the counter value read from the pixel data buffer (orzero if the reference voltage was not reached). This determination maybe performed in an integrated processor or in a processor external tothe sensor array. The determination may use the calculation${P_{i} = {C\frac{V_{ref}}{N_{i} - N_{0}}}},$

[0037] as described above.

[0038] An alternative embodiment of the method of the invention is shownin FIG. 5. Operation begins at start block 402. The pixel sensor isreset at block 404, by discharging the floating diffusion capacitor forexample. The output from the photo-detector is integrated at block 406.If a floating diffusion capacitor is used, the photo-detector outputcurrent is integrated by the capacitor to produce a photo-voltage. Atdecision block 422 the photo-voltage is compared with a referencevoltage. If the photo-voltage is greater than the reference voltage asdepicted by the positive branch from decision block 423, the currentvalue of a global counter (which is supplied to the pixel sensor) isstored into a pixel data buffer at block 410. At decision block 424, acheck is made to determine if the maximum time for integration has beenexceeded. If the maximum time has been exceeded, as depicted by thepositive branch from decision block 412, the pixel data buffer is readat block 414. If the maximum time has not been exceeded, as depicted bythe negative branch from decision block 424, the system waits until thetime has been exceeded. If the photo-voltage is less than the referencevoltage, as depicted by the negative branch from decision block 422,flow continues to block 412. Decision block 408 may be implemented as acomparator. Thus the value stored in the pixel data buffer will be thecounter value at which the photo-voltage reaches the reference voltage.At decision block 412, a check is made to determine if the maximum timefor integration has been exceeded. If the maximum time has beenexceeded, as depicted by the positive branch from decision block 412,the pixel data buffer is read. If the maximum time has not beenexceeded, as depicted by the negative branch from decision block 412,the system waits until the time has been exceeded. At block 414, thepixel data buffer is read, whether or not the reference voltage has beenreached. At block 416, the optical power is determined from the countervalue read from the pixel data buffer (or zero if the reference voltagewas not reached). The calculation may be performed by use of a lookuptable indexed by the counter difference N_(i)-N₀, or indexed by theintegration time calculated from the counter difference.

[0039] Another benefit of varying V_(ref) during the frame is thecapability of changing the range of light intensity levels accessed by aspecific set of successive digital codes representing integration times.This allows the sensitivity of the sensor to be changed. This isaccomplished by decreasing or increasing the average slope of V_(ref)for that set of codes to, respectively, increase or decrease the rangeof accessible intensity levels. This is illustrated in FIG. 6. FIG. 6shows a graph of voltage as a function of integration time. Theintegration time is represented by a digital code denoted by thevariable n. Light with intensity P₀ reaches the reference voltageV_(ref1) or V_(ref2) at a time denoted by the digital code m. If thevoltage reference is varied as a function of time in accordance with thecurve V_(ref1)(n), light with intensity P₁ reaches the reference voltageV_(ref1)(m+k) at a time denoted by the digital code m+k. On the otherhand, if the voltage reference is varied as a function of time inaccordance with the curve V_(ref2)(n), light with intensity P₂ reachesthe reference voltage V_(ref2)(m+k) at a time denoted by the digitalcode m+k. Hence, if the voltage reference is varied as a function oftime in accordance with the curve V_(ref1)(n), the digital codes m tom+k represent light with intensities from P₁ to P₀, while if the voltagereference is varied as a function of time in accordance with the curveV_(ref2)(n), the digital codes m to m+k represent light with intensitiesfrom P₂ to P₀. The average slope of the curve V_(ref1)(n) is greaterthan that of V_(ref2)(n), and consequently the sensitivity of a sensorusing voltage reference function V_(ref1)(n) will be higher.

[0040] Those of ordinary skill in the art will recognize that thepresent invention has been described in terms of exemplary embodimentsbased upon use of a capacitor to integrate the photo-detector output anda comparator to latch the global counter value into a pixel data buffer.However, the invention should not be so limited, since the presentinvention could be implemented using hardware or software componentequivalents to those described and claimed. Many other variations willalso be evident to those of ordinary skill in the art.

[0041] While the invention has been described in conjunction withspecific embodiments, it is evident that many alternatives,modifications, permutations and variations will become apparent to thoseof ordinary skill in the art in light of the foregoing description.Accordingly, it is intended that the present invention embrace all suchalternatives, modifications and variations as fall within the scope ofthe appended claims.

What is claimed is:
 1. A time-integrating pixel sensor, comprising: aphoto-detector; a capacitive element electrically coupled to thephoto-detector; a comparator element having a first input for receivinga reference voltage, a second input electrically coupled to thecapacitive element and an output; and a pixel data buffer responsive tothe comparator output and operable to store a digital code value.
 2. Atime-integrating pixel sensor in accordance with claim 1, furthercomprising: a voltage reference supply element, electrically coupled tothe first input of said comparator and operable to supply the referencevoltage thereto; and a global code generator, coupled to said pixel databuffer and operable to supply the digital code value thereto.
 3. Atime-integrating pixel sensor in accordance with claim 2, wherein saidvoltage reference supply element comprises a digital-to-analogconverter.
 4. A time-integrating pixel sensor in accordance with claim2, further comprising a processor operably coupled to said pixel databuffer and operable to convert said digital code value to an opticalpower level.
 5. A time-integrating pixel sensor in accordance with claim4, wherein said processor uses a lookup table stored in a memory.
 6. Atime-integrating pixel sensor in accordance with claim 4, wherein saidprocessor calculates the optical power level P in accordance with${P = {C\frac{V_{ref}}{N - N_{0}}}},$

where C is a constant, V_(ref) is the reference voltage, N is thedigital code value stored in the pixel data buffer, and N₀ is an initialdigital code value.
 7. A time-integrating pixel sensor in accordancewith claim 4, further comprising: a pixel data memory decoder coupled tosaid pixel data buffer.
 8. A time-integrating pixel sensor in accordancewith claim 2, wherein said voltage reference is varied in time.
 9. Atime-integrating pixel sensor in accordance with claim 8, furthercomprising a table memory for storing a lookup-table of voltagereference values wherein said lookup table of voltage reference valuesis accessed according to a lookup-table address generated by said globalcode generator and is operable to supply a voltage reference value tosaid voltage reference supply element.
 10. A time-integrating pixelsensor in accordance with claim 2, further comprising a table memory forstoring a lookup-table of digital code values wherein said lookup-tableof digital code values is accessed according to a lookup-table addressgenerated by said global code generator and is operable to provide thedigital code to said pixel data buffer
 11. A time-integrating pixelsensor in accordance with claim 1, wherein said pixel data buffercomprises a latch for each bit in said buffer.
 12. A time-integratingpixel sensor, comprising: (a) an array of pixel elements having aplurality of rows and a plurality of columns, each pixel elementcomprising: a photo-detector; a capacitive element electrically coupledto the photo-detector; a comparator element having a first input forreceiving a reference voltage, a second input electrically coupled tothe capacitive element and an output; and a pixel data buffer responsiveto the comparator element output and operable to store a digital codevalue; (b) a voltage reference supply element, electrically coupled tothe first input of said comparator and operable to supply the referencevoltage thereto; (c) a global code generator, operably coupled to saidpixel data buffer and operable to supply the digital code value thereto;(d) a pixel row decoder operable to select rows of pixel elements insaid array of pixel elements; (e) a pixel column decoder operable toselect columns of pixel elements in said array of pixel elements and toreceive data therefrom; and (f) a controller for controlling said pixelrow decoder, said pixel column decoder and said global code generator.13. A time-integrating pixel sensor in accordance with claim 12, whereinsaid global code generator generates a global code value and saiddigital code value is determined from said global code value.
 14. Atime-integrating pixel sensor in accordance with claim 13, wherein saiddigital code value is determined from a lookup table accessed by saidglobal code value.
 15. A time-integrating pixel sensor in accordancewith claim 12, wherein said global code generator generates a globalcode value and said reference voltage is determined from said globalcode value.
 16. A time-integrating pixel sensor in accordance with claim15, wherein said reference voltage is determined from a lookup tableaccessed by said global code value.
 17. A time-integrating pixel sensorin accordance with claim 12, further comprising: (g) an image processoroperable to receive data from said pixel column decoder.
 18. Atime-integrating pixel sensor in accordance with claim 12, wherein saidtime-integrating pixel sensor is formed as an integrated circuitsemiconductor device.
 19. A method for sensing the optical power oflight falling on a photo-detector having an input for receiving lightand an electrical output, comprising: operating a global code generator;integrating the electrical output of the photo-detector to obtain aphoto-voltage; comparing said photo-voltage with a reference voltage;storing the value of said global code generator into a pixel data bufferwhen said photo-voltage exceeds said reference voltage; and determiningthe optical power from the value of the global code generator storedinto said pixel data buffer.
 20. A method in accordance with claim 19,wherein said determining further comprises: reading the value of theglobal code generator stored into said pixel data buffer; and indexing alookup table by the value of the global code generator.
 21. A method inaccordance with claim 19, wherein said determining further comprises:reading the value of the global code generator stored into said pixeldata buffer; and calculating the optical power P in accordance with${P = {K\frac{V_{ref}}{t}}},$

where K is a constant, V_(ref) is the reference voltage and t is anintegration time determined from the value of the global code generator.22. A method for sensing the optical power of light falling on aphoto-detector having an input for receiving light and an electricaloutput, comprising: operating a global code generator to obtain a globalcode value; integrating the electrical output of the photo-detector toobtain a photo-voltage; determining a reference voltage value from saidglobal code value; generating reference voltage in accordance with saidreference voltage value; determining a digital code value from saidglobal code value; comparing said photo-voltage with said referencevoltage; storing said digital code value into a pixel data buffer ifsaid photo-voltage is greater than said reference voltage; anddetermining the optical power from the digital code value stored intosaid pixel data buffer.
 23. A method in accordance with claim 22,wherein said reference voltage value is a monotonically decreasingfunction of said global counter value.
 24. A method in accordance withclaim 22, wherein said reference voltage value is selected such that theoptical power of the light falling on the photo-detector is proportionalto the digital code value stored into said pixel data buffer.
 25. Amethod in accordance with claim 22, wherein said determining a referencevoltage value from said global code value comprises retrieving saidreference voltage value from a lookup table of reference voltage valuesindexed by said global code value.
 26. A method in accordance with claim22, wherein said determining a digital code value from said global codevalue comprises retrieving said digital code voltage value from a lookuptable of digital code values indexed by said global code value.
 27. Amethod in accordance with claim 22, wherein said digital code valuestored into said pixel data buffer is proportional to the optical powerof light falling on the photo-detector.
 28. A method in accordance withclaim 22, wherein said digital code value is equal to said global codevalue.
 29. A method for sensing the optical power of light falling on anarray of pixel sensors, each sensor having a pixel data buffer and aphoto-detector with an input for receiving light and an electricaloutput, said method comprising: for each pixel sensor of the array ofpixel sensors: integrating the electrical output of the photo-detectorto obtain a photo-voltage; measuring an integration time as the timetaken for said photo-voltage to reach a reference voltage; anddetermining the optical power from said integration time.
 30. A methodin accordance with claim 29, wherein said reference voltage is varied intime.
 31. A method in accordance with claim 29, wherein for each pixelsensor of the array of pixel sensors, said integrating comprisesintegrating the electrical output of the photo-detector in a capacitiveelement.
 32. A method in accordance with claim 29, further comprisingsetting said reference voltage level in accordance with a maximum of theoptical power falling on the array of pixel sensors.
 33. A method inaccordance with claim 29, further comprising setting said referencevoltage level in accordance with a minimum of the optical power fallingon the array of pixel sensors.
 34. A method in accordance with claim 29,further comprising: generating said reference voltage with a digital toanalog converter.
 35. A method in accordance with claim 29, wherein saidmeasuring comprises: running a global code generator; and for each pixelsensor of the array of pixel sensors: comparing said photo-voltage witha reference voltage; and storing the value of said global code generatorinto a pixel data buffer if said photo-voltage is greater than saidreference voltage.
 36. A method in accordance with claim 35, whereinsaid determining further comprises: decoding a row and column address ofa pixel data buffer in said array; reading the stored value of theglobal code generator from the pixel data buffer at said row and columnaddress; and indexing a lookup table by the difference between thestored value of the global code generator and an initial value of theglobal code generator.
 37. A method in accordance with claim 35, whereinsaid determining further comprises: decoding a row and column address ofa pixel data buffer in said array; reading the stored value of theglobal code generator from said pixel data buffer; and calculating theoptical power P in accordance with${P = {C\frac{V_{ref}}{N - N_{0}}}},$

where C is a constant, V_(ref) is the reference voltage, N is the storedvalue of the global counter, and N₀ is an initial value of the globalcode generator.